4-20mA Power source Question...

[cfordyce]

Which is better loop power source for 4-20mA singnals - 24VAC or 24VDC? Why?

 

[electricpete]

I think 24vdc. The 4-20ma loop will work on dc. Supplying ac only means that additional conversion would be required.

 

[schalk763]

The 4-20mA loop is usualy dc powered for the next reasons: -There needs to be a stable current flow for the different feedback levels ie DC.(In AC this changes all the time) - In AC the form factor of the voltage can change as the load impedance changes(This relates to different cable caracteristics)The problem is then that it places a lot of specifications on the power supply. -In DC this is not realy a problem. - In AC there is a sero crossing that equates to sero current flow. This means that you will have to have a DC offset to maintain the 4mA level.(4mA meaning that the loop is not broken.) The one problem with a DC 4-20mA is the posible risetime or change in level. If there are a high inductive reactance in the loop it will limit the rise time.- The other problem is tha cable capacitance. This will increase the capacity of the 4-20mA loop power supply. It realy all depends what you are measuring. If it is a tank level it will not change fast, where a position of a drum in a paper factory will change rapidly.

 

[cfordyce]

Thanks. I got it now.

 

[peebee]

In general, most all electronics are DC (there are exceptions). Most AC equipment has a DC power supply built into it.

 

[jbartos]

Suggestion: The 4-20mA loop instruments are usually designed for the dc loop current and associated dc power supply that is often 24VDC.